1. Field of the Invention
This invention relates to a method of and apparatus for compressing a discharged stencil in a stencil printer.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 2(1990)-175277 and 8(1996)-011416, it has been proposed to discharge used stencils from the printing drum into a discharged stencil box and store the discharged stencils in a compressed state in the discharged stencil box. By compressing the discharged stencils, a larger number of discharged stencils can be stored in the discharged stencil box.
FIG. 3 shows an example of the discharged stencil compression mechanism employed in the conventional stencil printer. As shown in FIG. 3, the conventional discharged stencil compression mechanism of this example comprises a stencil compression plate 11, a motor driven arm 12 which is supported for rotation coaxially with the stencil compression plate 11 and is connected to the stencil compression plate 11 by way of a spring 13, and a DC motor 18 which drives the motor driven arm 12 under the control of a CPU 16 by way of a motor driver IC. When the discharged stencils are to be compressed, the CPU 16 starts the motor 18 to rotate the motor driven arm 12 in the counterclockwise direction, whereby the stencil compression plate 11 is pressed against the discharged stencils pulled by the motor driven arm 12 stretching the spring 13. The CPU 16 detects the length of the spring 13 on the basis of signals from sensors (not shown) and stops the motor 18 when the length of the spring 13 becomes equal to the value corresponding to a predetermined compression torque. In this manner, the discharged stencils are compressed under a predetermined compression torque. The motor driven arm 12 is provided with an actuator piece 12a which turns off a limit switch 14 when the motor driven arm 12 is rotated over a predetermined angle, thereby limiting the maximum rotating angle of the motor driven arm 12. Further, the CPU 16 counts the number of the encoder pulses output from the rotary encoder 18, which represents the amount of rotation of the motor 18, thereby detecting the amount of compressed stencil in the discharged stencil box 15.
In the conventional discharged stencil compression mechanism, the load on the CPU can become excessive since the CPU must count the number of the encoder pulses output from the rotary encoder 18 and carry out sequence control for stencil making as well as detecting the compression torque. To employ a high-performance CPU conforming to the heavy load adds to the cost and is economically disadvantageous. Further, after a long use, the spring becomes weak and the relation between the length of the spring and the force applied thereto changes, which results in fluctuation in the measured compression torque and malfunction of the discharged stencil compression mechanism.
In view of the forgoing observations and description, the primary object of the present invention is to provide a method of and apparatus for compressing a discharged stencil which can accurately detect the compressive force on the discharged stencils without increasing the load on the CPU.
In accordance with a first aspect of the present invention, there is provided a method of compressing discharged stencils by a compression member driven by a DC compression motor to compress the discharged stencils, the method comprising the steps of detecting the cycle rate of pulses generated from a rotary encoder representing the speed of the compression motor and stopping the compression motor when the cycle rate of the pulses generated from the rotary encoder becomes equal to a value corresponding to a predetermined desired output torque of the compression motor.
As is well known, the rotary encoder is provided on an output shaft of the compression motor or a member driven by the output shaft of the compression motor, and the cycle rate of the pulses generated from the rotary encoder represents the speed of the compression motor. Since the compression motor is a DC motor, the output torque linearly changes with the speed (the number of rotations per unit time) of the motor, and accordingly, the output torque of the compression motor (proportional to the compressive force on the discharged stencils) can be detected from the cycle rate of the encoder pulses.
Since, for example, there is a time lag between the time the compression motor is stopped and the time the compressive force on the stencils is actually released, the time the compression motor is stopped need not be the end of the stencil compression step.
In order to easily and efficiently detect the cycle rate of the rotary encoder pulses, it is preferred that reference clocks be generated and the cycle rate of the rotary encoder pulses be detected by counting the number of the reference clock pulses generated in one cycle of the rotary encoder pulses.
In this case, that the cycle rate of the pulses generated from the rotary encoder becomes equal to the value corresponding to the predetermined desired output torque of the compression motor may be detected on the basis of the cycle rate of the rotary encoder pulses obtained by converting the number of the reference clocks generated in one cycle of the rotary encoder pulses to the cycle rate of the rotary encoder pulses, or may be detected directly on the basis of the number of the reference clocks generated in one cycle of the rotary encoder pulses without converting the number of the reference clocks generated in one cycle of the rotary encoder pulses to the cycle rate of the rotary encoder pulses.
In accordance with a second aspect of the present invention, there is provided a discharged stencil compressing apparatus for a stencil printer comprising a compression member driven by a DC compression motor to compress the discharged stencils, a rotary encoder which generates pulses representing the speed of the compression motor, a compression detecting means which detects the cycle rate of the pulses generated from the rotary encoder and generates a detecting signal when detecting that the cycle rate of the pulses generated from the rotary encoder becomes equal to a value corresponding to a predetermined desired output torque of the compression motor, and a control means which stops the compression motor upon receipt of the detecting signal from the compression detecting means.
The CPU for controlling the stencil printer may double as the control means of the discharged stencil compressing apparatus.
In order to easily and efficiently detect the cycle rate of the rotary encoder pulses, it is preferred that the compression detecting means be provided with a reference clock generator which generates reference clocks and an encoder pulse cycle measuring means which measures the cycle rate of the rotary encoder pulses by counting the number of the reference clock pulses generated in one cycle of the rotary encoder pulses.
In accordance with the present invention, since whether the compressive force applied to the discharged stencils reaches a predetermined value is detected on the basis of the cycle rate of the rotary encoder pulses, malfunction of the discharged stencil compression mechanism and/or measuring errors due to change of the relation between the length of the spring and the force applied thereto due to deterioration with time of the spring can be prevented.
Further, since it is not necessary to detect the length of the spring, the load on the CPU can be lightened, and accordingly, the discharged stencil compression mechanism can be produced at low cost.